The development of devices for delivering liquid medicament to a patient, have during the recent years become more and more directed towards the ability for the patient themselves to administer the medicament with a predetermined dose in an easy, safe and reliable way and also to facilitate the administration of medicaments for hospital personnel in the same facilitated way. Depending on the intended use and type of medicament, they have developed a varying degree of automatic functions.
Currently existing automatic medicament delivery devices, conventionally comprise a cartridge or the like, containing the liquid medicament to be delivered. Said conventional delivery device is further provided with a plunger rod that is adapted to be in contact with a piston provided inside the cartridge. Upon delivery of the medicament contained in the cartridge, the plunger rod will exert a force upon the piston, whereupon the piston will move forward inside the cartridge and thus expel the medicament from the cartridge. The distance that the piston moves inside the cartridge, determines the amount of medicament to be delivered.
The force that is applied to the piston during medicament delivery is generally accomplished by means of having a pre-tensed helical spring connected to the plunger rod and thus provided in the interior of the delivery device, wherein the force is obtained in accordance with Hooke's law (1):F=−k*y  (1)wherein F is the force exerted by the spring (N), y is the displacement of the spring from its original position (m) and k is the spring constant (N/m).
From Hookes law follows that the force acting on the piston will decline linearly as the piston moves forward in the cartridge. Thus, when a large volume of medicament is to be expelled from the cartridge, the force needs to be initially high in order to be able to move the piston all the way down to the required position of the piston in the cartridge. However, the conventional cartridge is often made of an easily breakable material, such as glass, and having an initial high force acting on the piston will result in that there is a substantial risk of damaging the cartridge, which is most undesirable.
Having for instance a high viscosity medicament contained in the cartridge or having a fine needle attached to the delivery device will also require a higher force to act on the piston. The same applies for situations when the medicament is to be delivered within a short period of time. One can generally say that when a plunger rod is allowed to freely act on the piston, there is a substantial risk of damaging the cartridge when the piston is applied with a force that is above or equal to approximately 50-60 N.
One solution to the problem is to provide the delivery device with a spring having a smaller spring constant, i.e. the gradient in the force-way diagram will have more flat appearance and the initial force acting on the piston will be decreased. However, a smaller spring constant would require a larger spring and hence a larger device. A larger device is generally not handled as easily as a device with a smaller size. Another problem is, that there is a minimum force value required to initially act on the piston in order for the piston to start the movement from its original position in the distal end of the cartridge, which minimum force in the art often is referred to as the “break loose force”. This force would not be obtained if the device was provided with a spring having a too small spring constant.
Also, the force acting on the piston is higher during the beginning of the medicament delivery procedure than towards the end, which results in that the piston moves faster in the beginning than in the end of said procedure, i.e. the medicament is during the procedure delivered to the patient at a higher rate in the beginning than in the end. This is undesirable, especially when the medicament is to be inhaled by the patient. This phenomena also results in that the rate with which the medicament is delivered may differ from one dose to another, since a higher dose requires an initially higher force to act on the piston than a lower dose, i.e. the so called “dose-to-dose accuracy” is poor with prior art automatic delivery devices.
Moreover, the conventional cartridge does not always have a smooth interior surface but may exhibit irregularities or unevenness as a result from the manufacturing procedure or as a result from the lubrication procedure, since the interior of the conventional cartridge most often is lubricated before use, for instance by the use of silicon oil. Such an irregularity or unevenness may increase the travel resistance acting on the piston which may cause the piston to slow down or even get stuck before it has reached its predetermined position inside the cartridge, especially if the irregularity is to be found towards the end of the distance that the piston is required to travel when the force acting on it has declined to a low value. It is generally known in the art that the force acting on the piston should not be below approximately 5 N, which thus is the lowest sliding force value needed in order not to allow the piston to get stuck before the entire set dose has been delivered.
Another problem is that the conventional delivery devices are generally made of plastic material due to manufacturing and economical reasons. Having a pre-tensed spring provided in the interior of such a device, results in that the tension caused by the pre-tensed spring, is held back by means of plastic components, which can lead to creep and hence plastic deformation of the plastic materials. This may reduce the life of the device and affect its accuracy and may also affect the automatic delivery function of said device. Also, having a high force acting on the piston during medicament delivery can cause damage of the plastic components of the device, which thus is another reason why it is not suitable to have a too high force applied to the piston, besides the risk of damaging the cartridge.
It is also important that the user of the delivery device is able to set the amount of medicament that is to be delivered in a relatively easy and reliable way. Likewise it is important and highly desired that such a delivery device is able to target specific time limits, for instance a predetermined injection time or deliver a dose within a determined time range.
Since these automatic medicament delivery devices have to be carried by the users in their pockets or their purses, it is also important that these devices are as smaller and shorter as possible.
U.S. Pat. No. 5,478,316 (Bitdinger et al) describes a device for automatic injection of a material into the body. In order to avoid the high impact of prior art devices, the device is provided with a constant force spring for moving a syringe assembly with respect to a housing and towards the skin of the patient, and for urging a rod in the direction of a piston provided inside a cartridge. The force exerted by the constant force spring is said to be sufficient to overcome the friction between the piston and the cartridge and between the needle and the user's skin.
Even though U.S. Pat. No. 5,478,316 describes the avoidance of a high impact, the device disclosed is not provided with means in order to set the force exerted on the rod to a predetermined force value, thus the advantage of applying a force to the rod that is within a predetermined force range is not described. Moreover, the device is not provided with means in order to set a predetermined dose of medicament to be delivered having a high degree of accuracy.
Devices which can be automatically expressed the dose on demand and offer a repeatable dose have been desirable the recent years. This has been described in:    The U.S. Pat. No. 5,104,380 patent by a syringe device comprising a body and a rotatable dose setting device mounted on the body and capable of being moved to a selected set position, a latch arranged to retain the setting device in the set position, and means arranged to release the latch to cause the set dose to be expelled. Movement of the dose setting device to the selected set position is accompanied by rotational straining of a spring, which, when the latch is released, provides the force for expelling the set dose. When the latch is released, the setting device is returned to an original position to drive a plunger through a one-way clutch to expel the set dose. The disclosed driving means comprises a quick pitch screw thread arrangement for transforming rotation of the setting device into linear movement of the plunger. The body is adapted for receiving a cartridge containing a fluid to be injected by having a cartridge container removable from the body for insertion of a cartridge and then removal of the cartridge container is arranged to release the quick pitch screw thread device thus allowing the plunger to be returned to an initial position; and    in the EP1 351 732 B1 patent by a medicine dosage setting device for use in combination with fluid medicament containers, wherein the dose setting device comprises a housing, a drive member, a spring means, a dose setting assembly and releasable latch means. The dose setting assembly comprises a dose setting member being moveable in a first direction to a selected set position against the bias of the spring means, wherein movement of the dose setting member is accompanied by straining of the spring. The dose setting member can also be moveable in a second direction to selectively adjust the set position. The releasable latch means are associated with the housing and adapted to retain the dose setting member in the set position against the bias of the spring means. Further the release of the latch means causes the dose setting assembly to drive the drive member by the spring means to thereby expel a set dose from a fluid medicament container when the dose-setting device is used in combination therewith.
However, not all these automatic medicament delivery devices offer the opportunity to expel a predetermined volume of medicament having a high degree of accuracy and show the user in a simple manner how much medicament is left after a dose or several doses have been expelled and at the same time offering a shorter device.